Im going to explain how I got WP7.8 installed in my HTC Radar.
Mode : Forced Update aka Cable trick
I had to do it like the below.
1. Disconnected WiFi and Mobile network (flight mode also worked during one of the tries, but with disabled WiFi) in my HTC Radar, Settings -> Wifi, Settings -> mobile network (toggle switches to OFF)
2. Connect via USB to Laptop, Start Zune, wait until your phone is detected
3. Had my internet connection via ethernet cable for the laptop
4. In Zune, Phone -> settings -> Update :
wait until it shows the phone is upto date 7.10 (8xxx or 7xxx)
5. Click any of the other items from the list (on the left side from the zune menu)
6. Once again click on the "Update", will show you checking for Updates in the screen
7. From the point of Click count upto 3 (or upto 4) -> corresponding to a wait time of 3 or 4 seconds
8. Disconnect the Internet Cable
Three possible outcomes:
Outcome a. Cannot connect to server : you have been disconnecting faster or the wait time was not sufficient. Reconnect the network once again repeat but wait for one more count more before you disconnect the cable
For 80072EFF error code, try once again from 1
For 80072F05 error code, the last update has over written your date and time settings (usually it sets to the year 1981 and probably the zune and Microsoft servers date mismatch disallows your server connection, until you change date and time to current day -> disable the auto update of date in settings and set it manually)
Outcome b. Your phone is up to date 7.10 (8xxx or 7xxx), you are too slow, may be count 1 less before you disconnect the cable.
Outcome c. You are lucky, continue reading from point 9!
9. The screen would still show checking for updates for 2 -3 seconds
10.Then the message would show, 7.10.xxxx update available for phone.
11. Also in HTC Radar a message appears saying that an update is available for your phone.
12. Attention !!! connect back your ethernet cable before you click on the update button from Zune (else you have to repeat from steps 1 to 11 !!!)
Repeat the above steps until you reach 7.10.8858 or WP7.8 version, as for each tries only intermediate or succeeding versions may be installed.
Disclaimer: I have shared my own experience of the update, which worked for me. I also do not share the risk arising like - bricking of your phone or corrupted OS or incorrect working of certain OS settings/apps/features, out of doing the above process. It involves lots of patience (for me i tried nearly 15 to 20 to reach WP7.8). The risk, as always, doing the cable trick, for your devices, lies with you !
Though, I have a feeling that the updates are reliable and are usually from Microsoft servers, I think the risk is miniscule. Unless you disconnect the USB of your phone in the process during the updates or you have a bad cable or a bad USB contacts resulting in fatal error of flashing. But always Zune provides you the option of restoring to the previous OS version !